The effect of elevated atmospheric CO2 on the vitamin C concentration of (sour) orange juice

Abstract Well-watered and fertilized sour orange trees ( Citrus aurantium L.) have been grown since the seedling stage (November 1987) out-of-doors at Phoenix, AZ, USA in clear-plastic-wall open-top chambers maintained at atmospheric CO 2 concentrations of 400 and 700 μl/l. Over the 12-year period 1988–1999, all fruit produced by each tree were counted and weighed; and over the last 8 years of this period (1992–1999), the concentration of vitamin C in the juice of the fruit was measured. Based upon these data, it has been determined that when the 75% increase in the air’s CO 2 concentration doubles fruit production, it increases the vitamin C concentration of the juice of the fruit by 7%. This nutritional enhancement is even greater in years when fruit production is more than doubled, due to the CO 2 -enriched fruit being slightly smaller than the ambient-treatment fruit in such circumstances, which increases the vitamin C concentration of the juice of the CO 2 -enriched fruit. In years when the CO 2 -induced increase in fruit production is less than 100%, however, the CO 2 -enriched fruit are generally slightly larger than the ambient-treatment fruit, which reduces the CO 2 -induced increase in juice vitamin C concentration below the basic 7% level characteristic of fruit of identical size. In what appears to be an approach to long-term equilibrium conditions, five of the last 6 years have seen the CO 2 -induced increase in the number of fruit produced hover at 74±9%, while the CO 2 -induced increase in fruit fresh weight has averaged 4±2% and the CO 2 -induced increase in juice vitamin C concentration has averaged 5±1%.